1. Field of the Invention
The present invention relates to a power steering system wherein power assistance derived therefrom will be modulated according to a vehicle operating condition such as vehicle speed.
2. Description of the Prior Art
In maneuvering a steering wheel of an automotive vehicle, it is recognized that power assistance at high vehicle speeds may desirably be less than at low speeds to allow for stabilization of the steering wheel at high speeds.
A power steering system with the above-mentioned steering characteristics is known having a reaction device which in accordance with fluid pressure supplied thereto provides resistance against relative rotation between valve elements of a servo-valve. The servo-valve controls fluid flow to and from a fluid motor according to the relative rotation between the valve elements, which, in turn, generates the power assistance. The fluid pressure supplied to the reaction device is to be controlled at a low level when high power assistance is necessary at low vehicle speeds, and on the contrary, at a high level when less power assistance is required at high vehicle speeds.
Conventionally, the fluid pressure supplied to the reaction device is controlled by a solenoid-operated control valve in accordance with vehicle speed and has no relation with the fluid pressure applied to the fluid motor.
In such prior art, as shown by the two-dot chain line in FIG. 10, the fluid pressure curve at high vehicle speeds corresponding to pressure supplied to the fluid motor with a manual torque applied to the servo-valve by means of a steering wheel is such that the fluid pressure curve at low vehicle speeds is parallelly shifted rightward by adding a certain amount of manual torque. Consequently, there is a problem in that variation of steering effort is poor while the steering wheel is turned at high vehicle speeds. To obtain a desirable variation of steering effort while turning the steering wheel at high vehicle speeds, the slope of the fluid pressure curve is expected to be gentle as shown by the full line in FIG. 10.